Conventional cellular phones provide an audio output which can be difficult to hear for a listener whose hearing is impaired. Increasing the output volume of the cellular phone is usually only partially effective when the listener's hearing is impaired. Typical hearing impairment occurs at select frequency bands. The hearing impairment may be complete or partial at any band. Uniform increasing of the output volume only addresses those bands which are partially impaired and so a uniform increase only partially aids the listener. In certain bands, which are completely impaired, the user still does not hear. The listener can also experience discomfort at the loudness of the output in bands which are not impaired in order to be able hear the other bands.
Conventional hearing aids typically provide selective amplification of sound to compensate for a user's specific hearing impairment.
Voice coder-decoders ("vocoders") have been used in cellular phones to achieve compression in the amount of digital information necessary to represent human speech. A vocoder in a transmitting device derives a vocal tract model in the form of a digital filter and encodes a digital sound signal using one or more "codebooks". Each codebook represents an excitation of the derived vocal tract filter in an area of speech. One typical codebook represents long-term excitations, such as pitch and voiced sounds. Another typical codebook represents short-term excitations, such as noise and unvoiced sounds. The vocoder generates a digital signal including vocal tract filter parameters and codebook excitations. The signal also includes information from which the codebooks can be reconstructed. In this way, the encoded signal is effectively compressed and hence uses less space than directly digitally representing every sound.
A receiving vocoder decodes a compressed digital signal using codebooks and the vocal tract filter. Based upon the parameters contained in the signal, the vocoder reconstructs the sound into an uncompressed digital sound. The digital signal is converted to an analog signal and output through a speaker.